Britons were put on alert by the Home Secretary last night for the 'real threat' of an al-Qaeda terrorist strike, as the anniversary of the World Trade Centre attack triggered an unprecedented security operation in major UK cities.

David Blunkett warned that the Islamic terror group would try to attack again, and that British citizens 'remain at risk' a year on from 11 September.

'The UK and its interests overseas remain a target for al-Qaeda terrorists who will almost certainly seek to carry out further attacks, drawing on the network of cells which they have already established,' the Home Secretary told The Observer. 'A real threat remains to this country. That is the stark truth.'

Intelligence chiefs have warned of the danger of a 'lone wolf' bomber - a fanatic inspired by Osama bin Laden, rather than directed by him - using the anniversary on Wednesday to launch an attack on Britain. Armed police will mount mobile roadblocks in London while businesses have been warned to tighten security and religious leaders advised on personal security.

Intelligence sources also fear a wave of false alarms - malicious or otherwise - which could wreak havoc among a jumpy public.

'There is concern that an opportunist might want to take advantage of the situation to gain extra publicity,' said one Whitehall source, warning of chaos if the public or the media over-reacted.

Scotland Yard Assistant Commissioner David Veness said people should adopt a 'business as usual' stance but remain vigilant.

Monitoring of those with extremist links has been stepped up, with Sheikh Abu Hamza, the controversial imam of Finsbury Park mosque in north London, claiming he is under 24-hour surveillance.

One police source said: 'The main threat is from lone actors or small groups doing things on their own initiative.'

The Ministry of Defence said the armed forces were ready to support the police, with air defence on 'constant alert'. In the US, airspace over Washington and New York is patrolled around the clock by war planes.

Air passengers are warned to expect delays on Wednesday due to rigorous searches, with armed police patrols in terminals stepped up. British Airways has cancelled a quarter of its transatlantic services for 11 September.

Blunkett said Britain was far better prepared than a year ago, but 'there is no such thing as 100 per cent intelligence and Britons remain at risk'.